1. Field of the Invention
The present invention relates to a hot-wire welding process, or more in particular to a method of control and apparatus for a hot-wire welding process in which magnetic arc blow and spatters are eliminated and maintain the heating power of the hot-wire at a proper value automatically.
2. Description of the Prior Art
FIG. 1 shows a configuration of a welding apparatus used conventionally for hot-wire TIG welding process.
A tungsten electrode 2 making up an arc electrode in the TIG welding torch 1 and a base metal 3 are connected to a welding arc power supply 4 directly, and an arc 5 is formed with the tungsten electrode 2 as a negative electrode in an argon shielding gas. A welding filler wire 6 is supplied from a wire feeder 7 through a conduit 8 and a contact tip 9 connected therewith to an arc forming section and is contacted with the base metal 3. The contact tip 9 and the base metal 3 are connected to a wire heating power supply 10. A direct or alternating current is supplied to the filler wire 6 thereby to generate Joule heat, thus improving the melting rate of the filler wire 6.
The optimum molten state of the filler wire 6 in the hot-wire TIG welding process is such that the filler wire 6 is completely molten immediately before entering a molten pool 11 and the molten metal continues to drop without interruption. In such a case, the metal is melted and the molten metal transfers to the molten pool 11 as if hot water is poured from a kettle, thus forming a superior bead 12.
For this purpose, it is necessary to control the filler wire 6 to be energized and heated in the extension e between the contact tip 9 and the base metal 3 in such a manner as to strike a balance between power supplied thereto and the wire melting amount.
In the hot-wire TIG welding process, however, it is conventionally known that with the increase in wire energization current, an electromagnetic force is generated with the arc current, which causes what is called "magnetic arc blow", making the welding difficult.
As a measure against this inconvenience traditionally known and employed from old days as a common technical knowledge, an arc current as high as possible is selected to increase the stiffness of the TIG arc and the wire heating current is set to less than one half of the arc current to heat the wire with AC rather than DC current. In order to increase the wire melting rate, however, it is necessary to increase the wire current. As a result, if a proper arc current is selected for a given object of welding to obtain a wire melting rate of, say, 20 g/min, a magnetic arc blow may occur continuously unless the arc is set to a very short length less than 1.5 mm, thus making the welding work difficult. This conventional method, therefore, has been limited in applications.
In the prior art, the power for heating the wire is controlled in such a manner that while the extension e is kept as constant as possible during the welding work, the wire melting conditions are observed by the welding operator to adjust the wire power in accordance with the wire feed rate, in sole dependence upon the intuition and labor of the welding operator:
In contrast, the U.S. Pat. No. 4,6l4,856 proposes a method of controlling the applied power to a value matching the wire feed rate by measuring the power applied to the extension e during the welding work.
Specifically, in view of the fact that the amount of heat necessary for melting the wire is proportional to the applied power in principle, the applied power is supplied in proportion to the wire feed rate.
As a result, by manually adjusting the above-mentioned proportionality appropriately, a great variation in extension e or a considerable change in wire feed rate in the welding can be met allowable in a considerable degree.
Even though proper conditions are determined in this way to keep subsequent conditions, the arc current, arc length or the angle of inserting the filler wire 6 into the molten pool 11 cause a change in the amount of heat transferred to the filler wire 6 from the arc 5 or the molten pool 11, so that the melting condition somewhat changes, thereby causing a deviation from the proper meling state.
If the applied power deviates toward an excessive side for the wire feed rate, a so-called spattering phenomenon takes place, and the arc 5 is disturbed with a current passing through the arc between the tip of the filler wire 6 and the base metal 3 or the tungsten electrode 2. These phenomena disturb welding operation very much. In the event that the applied power becomes too small for the wire feed rate, on the other hand, the apparent arc conditions remain substantially unchanged and the welding operator continues to proceed with his work without noticing the condition change, with the result that an unmolten wire 13 is left in the deposited metal 14 often forming a welding defect as shown in FIG. 2. To prevent these troubles, the welding operator is required to observe the molten part of the wire or the arc as frequently as possible during this welding work, checking to see whether the proper welding conditions have been kept to be met while adjusting the applied power. This decision as to whether the proper conditions are maintained depends on the intuition of the welding operator.